There are three ways to have a phone in more than one room.
Easiest: a cordless phone set
Connect a cordless phone base to Cell2Jack. Every cordless handset that goes with that base then works in whatever room you put it.
More than one Cell2Jack
Your cell phone can connect to more than one Cell2Jack. Put a Cell2Jack with a home phone in each room.
Through your wall jacks
You can feed your house wiring so every wall jack works. Only do this if you are sure the wiring is clear of the old phone service. Read using a wall phone jack first, because a live line will damage the device.
The maker says multiple home phones can ring at once, with no maximum stated. That is separate from cell phones: up to two cell phones can pair to one Cell2Jack.